1. Field of the Invention
The present invention relates to synthesis of materials using combustion in powder metallurgy and, more specifically, to a chemically activated combustion synthesis (CACS) of pure sub-micron powders of silicon carbide (SiC) used for production of abrasives, advance ceramics, composites, etc.
2. Description of the Prior Art
Silicon carbide has a unique combination of mechanical, electrical, thermal, and chemical properties, including high hardness and thermal conductivity combined with tailored electrical resistivity and outstanding corrosion resistance in certain chemical environments. These properties combined with high strength and high fracture toughness, make SiC-based ceramics an ideal material for a variety of applications, such as: abrasives; polishing plates for semiconductor wafers; advanced ceramics and composites (ceramics turbine, anti-radiation components, temperature sealing materials); ballistic armor tiles for personnel, vehicles, and aircrafts; electric and electronics products; high temperature piezoelectric sensors, resistors, semiconductors, and photonic thin films. In some applications, silicon carbide replaces metals, tungsten carbides and other ceramic materials, such as aluminum oxide.
One typical conventional method for synthesis of silicon carbide is based on a carbo-thermal reduction of silicon oxide. The reduction reaction SiO2+3C=SiC+2CO is conducted for hours in atmospheric air at temperatures about 2000° C. It is well recognized that under such conditions (i.e. high temperatures and long duration of synthesis) it is essentially impossible to produce pure silicon carbide powder with a particle size of less than 5 micron. There exist other methods for synthesis of fine SiC powders in laboratories, including plasma-chemical synthesis, microwave synthesis, reaction dispersion, chemical vapor deposition, and methods of wet chemical precipitation.
For the purpose of illustration, some methods and compositions for manufacturing silicon carbide powders known in the industry are described in the patent publications given below.
U.S. Pat. No. 4,226,841 issued in 1980 to Komeya, et al. relates to a method of producing a silicon carbide powder consisting of fine particles of uniform shape and size. The method is performed by baking a powdery mixture consisting of silica powder, carbon powder, and silicon carbide at a temperature of 1,350° C. to 1,850° C. under in a non-oxidizing atmosphere.
U.S. Pat. No. 4,676,966 issued in 1987 to Endo, et al. discloses a method for the preparation of a finely divided powder of silicon carbide as a promising material for sintered ceramic products of silicon carbide. The method comprises vapor-phase pyrolysis of a vaporizable organosilicon compound having, in a molecule, at least two, e.g. 2, 3 or 4, silicon atoms and at least one hydrogen atom directly bonded to the silicon atom but having no oxygen or halogen atom directly bonded to the silicon atom at a temperature of 750° C. or higher.
U.S. Pat. No. 4,832,929 issued in 1989 to Saiki, et al. discloses a process for producing a silicon carbide powder by reacting a gaseous silicon compound or granular silicon with a carbon compound at a high temperature. In the reaction, the amount of free carbon content in the resultant silicon carbide particles can be controlled by monitoring the amount of unsaturated hydrocarbon, such as acetylene, as a by-product. Moreover, silicon carbide particles can contain boron dispersed uniformly in the particles by a two-step process comprising first reacting a silicon source and a boron source without a carbon source in a first reaction zone to form boron-containing silicon particles, and second, reacting the resultant particles with a carbon source in a second reaction zone. Further, the above-mentioned monitoring of an unsaturated hydrocarbon by-product allows obtaining of silicon carbide particles containing no free carbon, and the silicon carbide particles containing boron in the particles.
U.S. Pat. No. 5,863,325 issued in 1999 to Kanemoto, et al. discloses a process for manufacturing high purity silicon carbide by using a high purity tetraethoxysilane or the like as the silicon source and a novolak-type phenol resin or the like as the carbon source. The process involves preparation of silicon carbide powder by calcining a mixture of the aforementioned sources in a non-oxidizing atmosphere and then post-treating the obtained silicon carbide powder at a temperature of 2000° C. for 5 to 20 minutes in order to obtain a silicon carbide powder having an average particle diameter of 10 to 500 μm and a content of impurity elements of 0.5 ppm or less. The obtained high-purity silicon carbide powder is intended for use in the manufacture of a silicon carbide single crystal having a decreased number of crystal defects.
U.S. Pat. No. 6,730,283 issued in 2004 to Konno, et al. discloses a method in which a fine β-silicon carbide powder is prepared by impregnating graphite with an organosilicon compound selected from crosslinkable silanes and siloxanes, causing the organosilicon compound to crosslink within the graphite. The powder is heated at 1,300° C. or higher in an inert gas stream for reaction. According to the authors, using only low-temperature heat treatment in air and high-temperature heat treatment in inert gas, the invention enables industrial, economical manufacture of fine silicon carbide powder in a stable manner.
U.S. Pat. No. 7,029,643 issued in 2006 to Otsuki, et al. discloses a method for manufacturing silicon carbide powder by sintering a mixture containing at least a silicon source and a carbon source, wherein a carbon source is a xylene-based resin. In preferable embodiment, the silicon source is an alkoxysilane compound. The silicon carbide powder produced by the above-mentioned method contains nitrogen in concentrations of 100 ppm or less and has volume resistivity of 1×100 Ω·cm or more.
U.S. Pat. No. 7,109,138 issued in 2006 to Bandyopadhyay, et al. discloses a composition for preparation of silicon carbide powder. The synergistic composition consists of a mixture of a source of pure silica such as silicon dioxide, a source of carbon such as activated charcoal, “beta” silicon carbide and a source of iron such as ferric nitrate. The cost effective synergistic composition is useful for the preparation of improved silicon carbide powder containing at least 90% SiC preferably rich in the β-phase. The silicon carbide powder of the aforementioned patent is intended for refractory and engineering applications.
U.S. Pat. No. 6,627,169 issued in 2003 to Itoh, et al. discloses a method for manufacturing a silicon carbide powder used for sintering silicon carbide body of high purity. The silicon carbide powder contains impurity elements, each of the impurity elements being contained in an amount of at most 0.01 ppm. The method includes a burning step, in which amounts of carbon monoxide generated during burning are detected and temperature adjustments are controlled in accordance with the detected amounts, and a heat treatment step, in which silicon carbide powder obtained in the burning step is heat-treated in a vacuum atmosphere; and the silicon carbide sintered body is formed by sintering silicon carbide powder.
The methods described above are based on sintering at high temperatures by consuming a large amount of external heat, which makes these processes expensive and energy-consuming. Therefore exothermic reactions for the production of silicon carbide are desirable. However, as is known to those skilled in the art, such exothermic reactions are difficult to obtain and control.
Furthermore, none of the above mentioned approaches allows effective production of sub-micron SiC powders with small amounts of impurities and with submicron size of particles. Thus, a need exists for an improved method for producing a sub-micron silicon carbide powder.